Bacteroides fragilis produces at least 8 different capsular polysaccharides. In order to facilitate purification of PSA from the other 7 capsular polysaccharides. Others have engineered a PSA producing strain employed mutation of a regulatory gene that controls the transcription of polysaccharide production genes, but left the actual genes that are required for all capsular polysaccharide synthesis intact. However, subsequent to the creation of this strain, it was shown that secondary mutations could cause reversion to polysaccharide production (1).